Alternating current electric motors are typically engineered to provide a maximum power (horsepower) output under load at a particular constant operating voltage. Unfortunately, during most of the life of such a motor, the motor is grossly underutilized and wastes a considerable amount of electric power. When such motors operate at anything less than a full load, any extra power provided to the motor is converted into waste heat by the windings of the motor. Induction motors are used in many applications such as refrigerators and air-conditioners, elevators, pool systems, boat lifts, washing machines, clothes driers, air compressors, etc. This type of motor is more reliable because it does not have brushes, is relatively quiet when operating and has predictable design performance characteristics.
Induction motors are less efficient when not fully loaded. A typical ⅓-horsepower induction motor draws about 6.6 amperes and exhibits a power conversion factor of about 80% under full load. This same motor has poor efficiency under a light load because of internal losses. Although the power factor may drop to 30% or so, the current consumed still remains high. Under no load, the same motor draws around 4.9 amperes; even though little or no useful work is being performed because there is no load on the motor.
It is well known that eddy current contributes to efficiency loss, particularly when operating under less than full load. This power loss is converted to heat, making the motor structure operate at a higher temperature, thereby, lowering the life expectancy of motor components such as bearings. Additionally, the heat enters the environment, requiring cooling of the area around the motor and contributing to heat build-up in buildings; an undesirable consequence when the buildings are being air-conditioned.
In lightly loaded induction motors, the rotor turns slightly faster than when it is heavily loaded, resulting in an increase in the stator inductance, resulting in a low power factor. This increase in rotational speed was measured in the prior art with devices such as tachometers and fed back into a motor control circuit. The circuit would then reduce power to the motor when the circuit detected that the motor was lightly loaded. By reducing the applied stator voltage the magnetic field is weakened and the rotor torque is lessened. If the voltage or power is decreased too much, slip, drag or stalling may occur. Therefore, reductions of the applied stator voltage or power must be controlled to provide sufficient voltage/power to prevent stalling and unsatisfactory operating characteristics such as vibration. These conditions can lead a reduction of the life of the motor.
High-permeability core materials also exhibit an abrupt “knee” where magnetic saturation occurs at a specific voltage. The operating point for the core material making up the motor's stator structure is established with a high flux density under normal line voltage. An increase in line voltage can bring about a large decrease in efficiency as magnetic saturation of the core material is approached. The increased line voltage creates only increased losses in efficiency rather than additional torque. Such losses tend to produce more heating, which in turn increases the losses by, for example, increasing the resistance of the windings.
When electric utility companies reduce line voltage (“brown-out”) during peak-usage periods, typical induction motors can fail by stalling or overheating. In such conditions where insufficient voltage is available for proper motor operation, it is better to not provide any voltage to the motor.
U.S. Pat. No. 4,806,838 and U.S. Pat. No. 4,823,067 reduce motor losses through the use of two separate parallel-acting run windings, one that has a higher impedance to produce a sufficient portion of field strength flux to operate the motor under partial load and the other has a lower impedance and is controlled to increase the field strength flux when the motor load increases. This requires modifications to the motor, including rewiring the winding of the motor; something not feasible for existing installations. It is desirable to obtain power savings through improved efficiency for induction motors that have a single run winding without making modifications to the motor.
What is needed is a system, method and apparatus of controlling power to an induction motor that will reduce power consumption and protect the motor from line voltage problems, overload, etc., without modifications to the motor itself.